The present invention relates to a method to controlling a highway frame phase of a time-division switch in a digital cross connect and digital exchanger and a time-division switch for variably controlling a frame phase.
Conventional digital cross connect and digital exchangers require a time-division switch which has a multi-switching function, i.e., a switching of channels simultaneously using a plurality of time slots. The multi-slot switching in turn requires that the time order of a plurality of time slots do not change after a switching operation, which is referred to as "Security of TSSI (Time Slot Sequency Integrity)".
Conventional system for achieving the security of TSSI for a time-division switch includes a double buffer system as described in a revised edition of "Digital Transmission Terms", edited and published Dy Electric Telecommunications Association, December 1988, page 74. The double buffer system has two data memories respectively provided with a single port. When data is being written into one of the two data memories, read-out is being executed from the other one. This state is maintained for one frame period equal to a switching period, thus exchanging one memory in a writing state and the other memory in a reading state at intervals of a frame.
In the conventional double buffer system, the two memories should be mutually switched between the writing state and the reading state simultaneously at the boundary of each frame, whereby the frame phase of an input highway of the time-division switch and the frame phase of an output highway of the same must be in a fixed phase relation with each other. Therefore, if the input highway is not in a fixed relation with the output highway or if a clock of the input highway varies with respect to a clock of the output highway, it is necessary to employ a buffer or an elastic store prior to connecting the input highway to a data memory to meet the frame phase of the input highway with the frame phase of the output highway, which results in a larger circuit configuration. Also, if a plurality of input highways comprised in a time-division switch have mutual phase variations, it is necessary to provide each of the input highways with an elastic store to cancel such clock phase variations, which also results in a larger circuit configuration. Further, in a time-division switch having a plurality of output highways, if the frame phases of the respective output highways are different from one another due to circuit configurations of the respective destinations connected thereto, it is necessary to provide each of the output highways with a buffer, which also causes a larger circuit configuration.